Not harmful at all, just a sign that you are overfeeding/underfiltering your tank, and not doing sufficient gravel vaccumings. They are likely planaria that come into your tank as micrscopic particles attached to your fish. In a clean tank, they die. In a tank with extra nutrients, the proliferate. Clean your tank more often, and they will go away.
BTW, you said they were clear? Are they worms, or look like jelly like anemones?
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